An example of a conventional display device using coherent light is shown in FIG. 4. A light source 50 contains a laser having a wavelength of 450 nm (blue), a laser having a wavelength of 520 nm (green), and a laser having a wavelength of 630 nm (red). These three laser beams are projected or scanned onto an image receiving part 51 using an optical element or a scanning unit to be displayed as an image. By using the laser beams having the wavelengths of 450 nm (blue), 520 nm (green), and 630 nm (red), respectively, the color gamut is expanded, and the color reproducibility is improved, whereby the image can be displayed in a color that is closer to the natural color. Further, reduction in power consumption can be achieved by using the lasers as the light source in contrast to the case of using a lamp, and further, the number of optical elements can be reduced because coherent light is employed, whereby miniaturization of the device can be expected.
However, when the laser beams as coherent light are projected onto the image receiving part to display an image, speckle noise occurs due to interference of the laser beams reflected at the image receiving part, and the speckle noise causes glittering of the image or bright and dark portions in the image. The principle of occurrence of speckle noise will be described with reference to FIG. 5. FIG. 5 is a lateral view of the image receiving part. Usually, the surface of the image receiving part 1 is not completely flat but has irregularities. A light beam 2 and a light beam 3 which are emitted from the light source 4 onto the image receiving part 1 are reflected at the image receiving part 1 and cross each other at a point A in FIG. 5. When phases of the respective light beams match at the point A (when a difference in light path lengths becomes equal to one wavelength), the light beams 2 and 3 interfere each other, and the light intensities are increased. Further, when the difference in light path lengths shifts by ½ wavelength, the light intensities are lowered. Because of speckle noise that occurs as described above, intensity spots appear in the displayed image, which result in glittering or bright and dark spots to human eyes. As a result, the quality of the projected image is degraded, which makes a viewer uncomfortable or tired, leading to considerable reduction in the product value.
As a method for avoiding such speckle noise, for example, a method of previously shifting the phase of a laser beam applied to the image receiving part has been proposed (refer to Japanese Published Patent Application No. 2003-98476: Patent Document 1). FIG. 6 is a schematic block diagram illustrating a laser display system for reducing speckle noise, which is disclosed in Patent Document 1. With reference to FIG. 6, the laser display system 100 comprises a laser 120, a beam expansion optics 124, a diffuser 134, and a beam shaping optics 138. The laser 120 emits a laser beam 122 of a desired wavelength continuously or pulsewise. The beam expansion optics 124 expands the laser beam 122 to generate a parallel beam 132 having a diameter required for filling an opening of the beam shaping optics 138. The expansion optics 124 includes a diverging lens 126 and a collimating lens 130. The diverging lens 126 converts the laser beam 122 into a diverging beam 128. The collimating lens 130 converts the diverging beam 128 into a parallel beam 132. The diffuser 134 is disposed between the laser 120 and the beam shaping optics 138 and driven by an operation assignment means 136. In the laser display system 100, the phase condition of the beam is made random using the diffuser 134, whereby an interference point on a screen 160 is made random to reduce speckle noise.
In the structure of the laser display system described in Patent Document 1, however, since the diffuser is employed, attenuation of optical output occurs when the light beam transmits the diffuser, and the attenuation during the transmission of the light beam becomes larger as the randomization of the phase condition by the diffuser is carried out with higher efficiency. Therefore, higher laser output is required. However, since it is usual that the output of a laser which is used as a light source for display is already considerably high (not less than 1 W), it is technologically difficult to increase the output power, and furthermore, large power consumption is required. An increase in the laser output becomes an obstacle to commercialization of a display device using a laser source.